The present invention relates to safety measures for lifting slings, including a safety method, a lifting sling and a safety tag to be attached to such lifting slings.
Lifting slings for connecting a lifting device, such as a crane hook, to a load to be lifted are well-known. Such slings are generally constituted by chains, wire ropes or synthetic fiber. Normally, the slings have two, three or four legs of equal length, which are joined together at their upper ends and are coupled to a common, elongated master link assembly, the master link being dimensioned so that it can be hung into the gap of a crane hook.
Each leg of the sling is normally equipped with a terminal attachment fitting, e.g. a hook, an end link or a shackle, for safe connection to the actual load. Alternatively, the lifting sling may be formed as a loop dimensioned to extend around and underneath the load during lifting. Such a loop can be endless, in which case it is directly hung into the crane hook or the like, or it can be connected to a master link assembly at the upper end portions of each leg, which extend between the respective upper end points and the two connection points making mechanical contact with the load during lifting.
For a long time, there has been an established practice that each sling leg, including any fittings at the upper and/or the lower ends of the legs, can be loaded up to a maximum static force for a particular type of sling, e.g. for a specific type of lifting chain (dimension, material quality grade, etc.). This maximum load is the rated Working Load Limit (WLL) for a lifting sling with only one leg (hanging down vertically during lifting).
For multileg slings, this maximum load or WLL is converted to another value for the whole sling. The converted value will depend on the number of legs and the inclinational angle to the vertical (hereinafter called the xe2x80x9cangle to verticalxe2x80x9d) for the legs.
The larger the angle to vertical is, the lower the WLL will be. For safe use, it is normally required that the sling has an attached sling tag marked with the particular WLL for the sling. A common practice has been established, whereby the WLL is rated at the angles of 45 degrees and 60 degrees to the vertical, always under the assumption that the load is symmetrical and equally distributed to the separate legs, i.e. that the angle to vertical is the same for each leg of the sling.
Specific Working Load Limits (WLL) for single chain slings, two-leg chain slings, three- and four-leg chain slings, and endless chain slings (loops) are given in the European Norm EN 818-5:1999 for angles up to 45 degrees and angles between 45 and 60 degrees, respectively. This document also includes examples of Identification tags for such chain slings (Annex D).
However, it has been experienced on the jobsites that it is very difficult to accurately measure the actual angle to vertical. Also, the measuring procedure has to be repeated for every load to be lifted.
If the Working Load Limit (WLL) of a one-leg sling is 1 ton (t), then the corresponding values for a two-leg sling can be easily calculated as follows (corresponding values can of course be calculated also for slings with three or four legs):
As indicated above, the sling tags will indicate the rated values for 45 and 60 degrees. Accordingly, in this case the actual angle is 50 degrees instead of 45 degrees, this measuring error of only 5 degrees will result in an overload on the sling legs of 1.414/1.286-1 which is approximately 10%. On the other hand, if the actual angle is 65 degrees instead of 60 degrees, this measuring error, likewise of only 5 degrees, will result in an overload of 1.000/0.845-1 which is approximately 18%.
Therefore, the measuring errors of the angle to vertical for slings in practical service will either result in overloads up to about 20% or even more, or make it necessary to derate the lifting capacity of the particular lifting sling so as to avoid such overloads.
The main object of the present invention is to achieve an improved and more accurate method for providing an acceptable safety measure when lifting a load by means of a lifting sling coupled to a lifting device as discussed above.
It is also an object of the invention to provide a simple practical means for implementing such an improved method.
The above-mentioned main object is achieved by introducing the concept of a horizontal length limit (HLL), which is a predetermined maximum limit of the horizontal distance between connection points, which for said load and said sling will result in said static force which may be applied to each sling leg during lifting. In this way, it is no longer necessary to measure the angle to the vertical of the sling legs. Instead, one has to check, in addition to observing the working load limit (WLL), that the maximum horizontal distance between any two connection points does not exceed the given horizontal length limit (HLL). Such a check or measurement is much easier to perform on the jobsite.
The expression xe2x80x9cmaximum limit of the horizontal distance between connection pointsxe2x80x9d means that the limit is related to the greatest distance between the connection points. In a rectangular configuration (4-leg-sling), the greatest distance is along a diagonal. Moreover, for loads having a complicated or irregular geometry, it may happen that the connection points are not located exactly in the same horizontal plane.
Apart from the improved method, as defined in claim 1, the invention also concerns a lifting sling (claim 8) provided with a safety indicator, such as a tag indicating the working load limit (WLL) as well as the horizontal length limit (HLL), and such a tag (claim 10) to be attached to the associated lifting sling.
The invention will explained further below with reference to the appended drawings.